Hydrogen chloride which is formed on a large industrial scale, for example, in chlorination reactions as a by-product usually contains impurities which interfere in its reuse. Therefore, numerous procedures for purifying hydrogen chloride are known, which, depending on the type and amount of impurities, can be used alone or in combination with other procedures [cf. "Ullmans Enzyklopadie der technischen Chemie" (Ullmans Encyclopedia of Industrial Chemistry), volume 20, 4th edition 1981, page 369 ff.].
Thus, for example, gaseous hydrogen chloride containing as impurities water and organic and/or inorganic substances, such as chlorine, chlorinated hydrocarbons, metal halides and hydrogen fluoride can be purified by a combination of different procedures (cf. DE-A 1,467,074). In this purification, the crude gas which is compressed to 0.4 MPa is cooled to about -10.degree. C. by means of a liquid coolant by counter-current contact and water and some of the impurities which are condensable under these conditions are removed. The virtually anhydrous crude gas is then compressed to 3 MPa and then contacted under pressure in a fractionating column by a counter-current process with liquid hydrogen chloride under reflux conditions. This removes impurities having a higher boiling point than hydrogen chloride in the lower part of the column, and purified hydrogen chloride is discharged at the top of the column. In the first compression stage, compressors containing concentrated sulfuric acid as sealing fluid, which acts as a drying agent and also contributes to the removal from the crude gas of any traces of water still present, are recommended. In the second compression stage, compressors of any desired construction having two or more stages and intermediate cooling can be used. Since the crude gas is virtually anhydrous, it is possible to use steel equipment not only for the second compression stage but also for the distillation under pressure, since it is essentially resistant to corrosive attacks, if the given temperature conditions are observed.
Gaseous hydrogen chloride containing as impurities volatile carbon compounds, such as hydrocarbons, chlorinated hydrocarbons and acid chlorides derived from carboxylic acids, such as phosgene, can be compressed directly as a crude gas without pretreatment and subsequently distilled, with a low-temperature condensation stage in between (cf. DE-A 2,143,994). This publication proposes to use two-or three-stage screw compressors for the compression and steel as the construction material.
The purification of hydrogen chloride formed in silane- or siloxane-producing plants and containing silanes, such as chlorosilanes, alkoxysilanes or alcohols as impurities is in general carried out by washing the gas mixture with a washing liquid, such as water (cf. DE-B 1,185,593), hydrochloric acid (cf. EP-A 114,226 and DE-A 3,533,577) or sulfuric acid (cf. DD-A 227,685). The more or less dilute acids formed in this process as by-products have to be concentrated by additional measures or disposed of by other means, since their reuse for synthetic purposes is very limited.
Since in a large chemical plant, crude gases containing hydrogen chloride usually originate from different synthetic areas and thus contain different impurities, it makes sense, for reasons of economics and environmental protection, to find a common process for their purification or processing.
Crude gaseous hydrogen chloride is formed, for example, in silane-producing or processing plants, such as in the preparation of silanes containing phenyl or phenylmethyl groups by reaction of H-silanes with chlorobenzene, the preparation of tetraethoxysilane by reaction of silicon tetrachloride with ethanol, the preparation of organoalkoxysilanes by reaction of organochlorosilanes with alkanols, such as methanol and ethanol, in which the corresponding alkyl chloride is formed as a by-product, and the preparation of organoacyloxysilanes by reaction of chlorosilanes with carboxylic acids, such as acetic acid, in which the corresponding acyl chloride, such as acetyl chloride, and acetic anydride are formed as by-products.
The crude gases which are formed therefore also contain, in addition to the starting materials and final products of the paricular reaction, the by-products which are formed in the reaction, and also water and any organic solvents which may have been used, such as ethyl acetate, acetone, toluene and xylene, just to mention the most important ones.
The type of potential impurities in the crude gas can be illustrated in more detail by the following examples: In the preparation of methyltriethoxysilane by reaction of methyltrichlorosilane with ethanol according to the equation EQU MeSiCl.sub.3 +3EtOH.fwdarw.MeSi(OEt).sub.3 +3HCl (1)
the resultant crude hydrogen chloride gas additionally contains, in addition to the starting materials and the final product, hydrocarbons which are present in ethanol as denaturants (about 2 percent) and ethyl chloride, which is formed in a side reaction according to the equation EQU EtOH+HCl.fwdarw.EtCl+H.sub.2 O (2)
In addition, mixed chloroethoxysilanes are present which can react with excess hydrogen chloride with the elimination of ethanol according to the equation EQU MeSiCl.sub.3-x (OEt).sub.x +y HCl.revreaction.MeSiCl.sub.3-x+y (OEt).sub.x-y +y EtOH (3)
(x=0, 1, 2, or 3 and y&lt;x) and the water of the reaction formed according to equation (2) can form gel-like methylsilicic acids with the mixed chloroethoxysilanes catalyzed by HCl according to the equation EQU n.MeSiCl.sub.3-x (OEt).sub.x +n.1.5H.sub.2 O.increment.(MeSiO.sub.1.5).sub.n +n.(3-x) HCl+n.x EtOH. (4)
Therefore, it is an object of the present invention to provide a process for purifying crude gaseous hydrogen chloride containing impurities. Still another object of the present invention is to purify crude gaseous hydrogen chloride containing impurities such as silanes containing chlorine and/or organic radicals, water alkanols, hydrocarbons, chlorinated hydrocarbons, organic acids and derivatives thereof. A further object of the present invention is to provide a process for purifying crude gaseous hydrogen chloride containing impurities by a combination of procedures consisting of condensation, compression and fractional distillation.